Bug 782676: require fontconfig 2.12.4 on Windows

Fontconfig has a new slowness issue, this time apparently because of locked cache files, preventing a successful cache update. See: https://bugs.freedesktop.org/show_bug.cgi?id=99360 Furthermore the slowness apparently happens at every GIMP startup, and also even when GIMP closes according to various reports. So let's bump Fontconfig version (for Win32 builds only).

Also let's not make it a recommendation anymore, but a requirement. That makes quite a few Win32-specific fontconfig bugs which have been fixed, and most are quite serious so this should not be considered as optional anymore, I guess.